Natural Swimmer: At level 1, you can swim at the same speed you can move on land, and hold your breath for a number of minutes equal to your Constitution score. You have webbed feet.

Water Breather: At level 2, gills open on the sides of your neck. You can now breathe freely underwater.

Innsmouth Look: At level 3, your eyes become huge and bulging. You can now see perfectly even in very low light (although not in total darkness).

Teeth and Claws: At level 4, your teeth become long and sharp, and your hands warp into claws. You can do 1d4 damage in melee even when unarmed.

Oceanic Adaptation: At level 5, your skin becomes rough and scaly, and your body bulks out with the masses of blubber and muscle required for existence in the ocean depths. You gain +1 AC, and inflict +1 damage in melee due to your increased bulk and strength. By this point you look like a total freak, and may want to consider investing in some very big hats and coats.

Blessing of Hydra and Dagon: From level 6 onwards, you gain spell-casting abilities equal to those of a Cleric of half your level, rounded down. (You don't gain any other cleric abilities: just the spells.)Lord of the Seas: At level 7, you gain the ability to command the creatures of the swamps and seas. This functions as per the Chaotic cleric ability Command Undead, except it applies to aquatic and amphibious creatures instead of undead. This ability does work on intelligent creatures such as Bullywugs, but they get a save vs. spells to resist it.

Command the Formless Spawn: At level 8, your Lord of the Sea ability also allows you to command unintelligent slime and ooze monsters.

Call the Cult: At level 9, you intuitively reach out through your dreams to nearby unstable individuals, calling them together to form a cult of which you are the leader. Over the next few weeks, 3d6 crazy people (a mixture of 1st-level fighters, thieves, and magic-users) will arrive, and serve you with fanatical loyalty. Whenever one dies, an equally crazy replacement will turn up 1d3 weeks later. Each time you go up another level you gain another 1d6 cultists.

The Sea is Calling: At level 10, you hear a song in your dreams that calls you to a holy place in a sunken city at the bottom of the sea. If you follow it, and complete the hazardous journey successfully, you undergo a final metamorphosis in which you shed the last remnants of your humanity. You cease to age, become immune to disease, and will live forever unless killed by violence.

no more boring fighters? why be a fighter when you could be a deepone!

possibly i would make armour increasingly useless or inhibit swimming or something to balance or mention most ppl horrified by you and want you dead or burned, possibly some obligations or fight other races or cults. Deepone-dolphin hybrids and weird followers might be cool

1) They need more XP per level.2) They increasingly need to conceal their true nature as they become more and more mutated.3) Their cult is kinda rubbish compared to the army that fighters get at name level.

I'd have thought that heavy armour would also be incompatible with swimming unless you were crazy strong. And, yes, I kinda assumed that most people would react negatively to an undisguised high-level hybrid. ('Aaah! Freaky fish-monster! Run away!') But you could always jack the XP-per-level up to Elf levels if you felt it was a bit too overpowered!